The invention disclosed herein addresses itself to the problem of correcting the deflection of the electron beams produced by the color guns of a color cathode ray tube (CRT) so that the beams will coincide and strike the same phosphor triad on the face of the CRT. Improper convergence is undesirable as it detracts from the definition of the images displayed.
The display of alphanumeric information produced by digital data systems utilizing CRTs is limited by the inability to represent the data in multi-color formats. While a myriad of techniques are available for producing color images, the use of a color CRT display provides many advantages. The availability, design and economic considerations, among others, make a CRT a more desirable candidate as the means for displaying digital information. The present invention may also be used in other applications using multi-gun CRT's, but would probably required some design changes to provide the requisite address information to the control circuitry taught by the present invention.
Color CRT's typically utilize three color guns each of which generates an electron beam that excite the respective red, blue or green phosphors coated on the face of the CRT. These beams are deflected magnetically so as to cause the beams to pass through the minute holes of a shadow mask, which is positioned between the guns and the phosphor face of the CRT. Correction coils provide corrective deflections to each electron beam prior to the deflection of the beams by the deflection yoke so that each of the beams will pass through the same hole in the shadow mask and strike its respective phosphor dot in the phosphor triad, and thereby produce a chromatic spot on the screen. The color of the spot will depend on the intensity of the respective electron beams when they strike the phosphor triad. These correction coils are required since the three color guns are positioned differently and therefore require different amounts of correction to cause the respective gun's electron beam to pass through the same hole in the shadow mask.
Presently the correction signals are generated by pulse shaping circuits. These circuits utilize inductors, resistors, capacitors and diodes to control the amplitude and overall waveshape of the correction signals. There is no independent correction control over the precise waveshape for the individual guns, however. Only the composite of the correction signals for each of the guns can be adjusted to achieve an overall convergence, and there is no control for the convergence at specific points on the CRT phosphor. The convergence signals interact so that adjustment for one gun affects the others.
The adjustment procedure further requires an interative approach. Convergence is set for the center of the CRT and then for the sides, but adjusting one color on one side may effect another color on the other side. A several step, best approximation procedure is therefore employed which is tedious and cumbersome.
Present systems also require the use of two convergence coils for each gun, one for horizontal correction and the other for vertical correction. The present invention however, requires the use of only one convergence coil for each color gun, and operates so as to provide individual control over the convergence correction for each electron beam, over the entire face of the CRT. The correction signals to each electron beam are independent of each other, and the individual beams can be additionally adjusted for each of the four quadrants of the face of the CRT.